Direct Digital Access (DDA) is a method by which Customer Premises Equipment (CPE) may obtain dial-up data connections from a digital switching network (hereafter referred to as a digital switch) at speeds up to 56 Kilobits per second (Kbps). The local loop, which comprises the CPE and metallic telephone wires, is linked to the digital switching network via a Direct Digital Access Office Channel Unit (DDA/OCU). Presently, DDA/OCUs are located at telephone company offices. At the telephone company office, the DDA/OCU is placed in a digital channel bank and may be connected directly to the digital switch via one trunk line of a T1 carrier. Alternatively, the DDA/OCU may be connected to the digital switch through a Direct Access and Cross-connect Switch (DACS) or a DS-0 Cross Connection.
The T1 carrier has 24 trunk lines. The digital switch communicates over the trunk lines using a time division multiplexed digital signal format. The digital signal consists of 24 eight-bit bytes, multiplexed as one byte per trunk, plus a framing bit for each frame of the digital signal. The framing bit is used to identify where the byte allocated to each of the 24 trunks are located on the T1 carrier. The framing bit has a pattern which takes twelve frames to repeat. These twelve frames, numbered one through twelve, are termed a superframe. Normally, all eight bits of each byte transmitted during a frame contain encoded voice or digital data information. However, in every sixth frame the least significant bit is replaced with a signalling bit by the digital switch. These signalling bits are used between the digital switch and the CPE to indicate to the CPE the progress of communication during a call setup period. Thus, the sixth and twelveth frames for each superframe include a signalling bit in the eight bit position of each of the 24 bytes.
Currently, DDA/OCUs perform two conversion functions which allow a CPE access to the digital switching network. The DS-1 to DS-0 conversion function inserts and extracts the appropriate byte per frame allocated to a trunk line and provides a 64 Kbps DS-0 interface dedicated only to that trunk line. The OCU portion of a DDA/OCU converts the DS-0 signal to a 56 Kbps signal for transmission to the CPE.
The DDA portion of a DDA/OCU selects the eighth bit in the sixth frame of each superframe being transmitted by the digital switch and extracts the signalling bit. The state of the selected signalling bits is interpreted by the DDA to indicate an on-hook or off-hook condition, i.e. "system mode". By convention, if the signalling bit is a ZERO, which signifies on-hook, the DDA sends a control mode idle (CMI) signal to the CPE. If the signalling bit is a ONE, which signifies off-hook, it forces the eighth bit of every byte sent to the CPE to a ONE and allows the first seven bits to pass to the CPE.
The DDA also monitors the trunk line for loopback sequences signifying a maintenance situation. A loopback sequence begins with at least four consecutive loopback control codes, each having bit eight equal to a zero, and ends when loopback control codes have not been detected for four consecutive bytes. After receiving a loopback sequence, the DDA/OCU looks for loopback commands which consist of alternating bytes of loopback control codes and pseudorandom data having bit eight equal to one. Once a DDA/OCU detects a loopback command, the DDA/OCU must go transparent; whereby it does not interfere with any communications on the trunk line directed to the CPE. However, there are many problems currently involved with maintenance testing situations due to limitations in existing DDA/OCUs which make it difficult for a DDA/OCU to function properly in maintenance situations.